A spark plug used for ignition in an internal combustion engine such as a gasoline engine or the like includes a metallic shell for attaching the spark plug to an engine head. The metallic shell has a generally tubular shape. In a state in which a ceramic insulator having a center electrode is inserted into the metallic shell, a crimp portion of the metallic shell is crimped, whereby the metallic shell is assembled to the ceramic insulator (for example, refer to Japanese Patent Application Laid-Open (kokai) No. 2002-164147).
In internal combustion engines, pressures in the combustion chambers thereof have been increased due to higher degree of supercharging and higher compression ratio. Therefore, the ceramic insulator of a spark plug is pressed with a larger force in a direction from a forward end side of the spark plug (the side where a spark gap is formed) toward a rear (proximal) end side thereof. As a result, there is a possibility that the ceramic insulator comes off the metallic shell. To solve this problem, it has been desired to enhance the ceramic insulator retaining performance of the metallic shell.
The present invention has been accomplished to address the above-mentioned problem and can be realized as the following modes.